An electronic ballast is a device which applies a high voltage across a fluorescent lamp to initiate an arc and start the lamp, and to limit current through the fluorescent lamp after the arc is initiated.
Most electronic ballasts include a output section including tuned LC resonant elements to achieve these functions. Some ballasts include input sections incorporating passive or active power factor correction.
An electronic ballast should provide high frequency high voltage supply to start the fluorescent lamp. After the lamp is started, it is possible that the power delivered to the lamp can be regulated by the ballast. In order to achieve this function, a substantially large DC input voltage is required, which results in the need for more expensive components with higher ratings or more complex control circuitry than if power regulation is not required.
Recently, there has been a push to use fluorescent lamps in residential applications, where incandescent lamps have typically been employed, in order to save energy. For example, commercially available PL lamps having screw in (or "Edison") bases can be used to replace incandescent lamps. These PL lamps typically have ratings of between 8 to 25 Watts. A low Watt fluorescent lamp provide illumination comparable to a higher Watt incandescent bulb, and thus saves energy. These PL lamps include a fluorescent tube and an electronic ballast circuit all in one screw in package. One commercially available PL lamp is a Philips Earth Light, model number SL 17/27. Because the electronic ballast circuitry in these lamps is disposable with the fluorescent tube when the tube burns out, it is desirable to keep the cost of the circuit components in the ballast low. A problem with these PL lamps is that they produce significant harmonic distortion, which results in increased costs to utility companies. Another problem with these PL lamps is that they have low power factor ratings; e.g., less than 0.7.
Another fluorescent lamp adapted for home use is a two pin fluorescent lamp which is used with a PL lamp adaptor, which adaptor contains a ballast circuit. An example of a commercially available two pin fluorescent lamp is a Philips model PLC15MM. An example of a commercially available adaptor for such a two pin lamp, containing a ballast, is a Lights of America PL lamp adaptor model 550 g.
Most prior art patents relating to electronic ballasts disclose circuits employing what is known as a boost mode of operation., wherein output voltage is always higher than input voltage, or passive L-C circuitry for power factor correction. For example, U.S. Pat. No. 5,001,400, issued to Nilssen on Mar. 19, 1991 and incorporated herein by reference, discloses boost mode power factor correction circuitry. U.S. Pat. No. 4,985,664, issued to Nilssen on Jan. 15, 1991 and incorporated herein by reference, discloses a forward converter topology. U.S. Pat. No. 5,258,692, issued to Jones on Nov. 2, 1993; U.S. Pat. No. 5,148,360, issued to Nguyen on Sep. 15, 1992; U.S. Pat. No. 5,057,749, issued to Nilssen on Oct. 15, 1991; U.S. Pat. No. 5,012,161 issued to Borowiec et al on Apr. 30, 1991; U.S. Pat. No. 5,099,407, issued to Thorne on Mar. 24, 1992; U.S. Pat. No. 5,180,950, issued to Nilssen on Jan. 19, 1993; and U.S. Pat. No. 5,004,947, issued to Nilssen on Apr. 2, 1991, all of which are incorporated herein by reference, all disclose passive correction.
Ballasts including self oscillating output circuits are known in the art.